The present invention concerns thermal spray powders, their production and use. Specifically, the invention concerns coating of aluminum substrates with thermally sprayed metal powders.
Different methods for producing coatings on aluminum substrates are previously known. These methods are used in, e.g., aluminum engine blocks having cylinder liners which are formed by thermal spraying.
U.S. Pat. No. 2,588,422 discloses an aluminum engine block having cylinder liners which are formed by thermal spraying. These liners are built up in two layers on the untreated surface of the engine block, the top layer being a hard slide layer such as steel about 1 mm in thickness and the lower layer being a molybdous interlayer about 50 microns in thickness. The interlayer, containing at least 60% molybdenum, does not constitute a slide layer, but is necessary in order to bind the hard slide layer to the aluminum block. Preferably, the interlayer is made up of pure molybdenum. The slide layer is a layer of hard metal, as for example carbon steel, bronze or stainless steel, in which the steel may be an alloy containing nickel, chromium, vanadium or molybdenum, for example. In principle, this two-layer structure provides a good slide layer, but the cost of the double coating is substantial.
In recent thermal spraying methods, the thermal spray powders are made up by a mixture of powdered steel with powdered molybdenum, such as described in the U.S. Pat. No. 6,095,107. The risk of segregation due to differences in properties between the base steel powder and the powder of crushed molybdenum is however a problem which may result in non-uniform coatings. Another disadvantage is that comparatively large amounts of molybdenum are required due to the segregation effect.
A main object of the present invention is to provide an inexpensive metal powder for thermal coating of substrates, especially for aluminum.
Another object is to provide a powder, which does not segregate and wherein the amount of expensive molybdenum alloying metal can be reduced in comparison with currently used methods.
A further object is to provide a thermal powder, which has high deposition efficiency and gives excellent coating quality.
Another object is to provide a thermal powder giving coatings of suitable porosity and oxide content and wherein the pores are predominantly closed, isolated and have an advantageous range of pore diameters.
These objects are obtained by a metal powder comprising a pre-alloyed iron base powder having particles of molybdenum, such as reduced molybdenum trioxide, diffusion alloyed to the particles of the base powder.